The invention relates to a polyisocyanate compound and a process for producing the same. In particular, it relates to a novel polyisocyanate compound useful as a starting material for an optical material, and a process for producing the same at good efficiency. It also relates to an optical material and an optical product using the same. Specifically, it relates to an optical material which has excellent optical characteristics such as a high refractive index, a low dispersion, an excellent transparency, lack of optical distortion and the like, and which is also good in solvent resistance and weatherability, and to an optical product formed of this optical material, such as a lens, a prism, optical fibers, a substrate for a recording medium, a filter, a glass, a vase and the like.
Recently, plastic materials have been used for optical parts in place of inorganic glass, especially for eyeglass lenses, because they are lightweight and have good dyeability and good impact resistance. In particular, a polyethylene glycol bisallylcarbonate polymer (hereinafter referred to as CR-39) and polymethyl methacrylate (PMMA) have been used, primarily for plastic lenses. However, it has a refractive index of 1.50, which is lower than that of inorganic glass. Therefore, when they are used as lens materials, for example, the thickness of the lenses has to be increased with the increasing power. Consequently, not only is the superiority of plastics as lightweight material been impaired, but also they have not been preferred in view of the aesthetic property. Furthermore, the concave lens, in particular, has had problems when the thickness (edge thickness) of the surrounding portion of the lens is increased causing a birefringence or a chromatic aberration to occur.
For these reasons, in order to be able to decrease the thickness of the lens while applying most of the characteristics of plastics having a low specific gravity, a plastic material having a high refractive index has been in demand. As a material having such a performance, for example, (1) a polymer comprising a xylene diisocyanate compound and a polythiol compound (official gazette of JP-A-63-46213), (2) a resin comprising an aliphatic linear sulfur-containing diisocyanate and a polythiol compound (official gazette of JP-A-2-153302), (3) a polymer comprising a dithian derivative substituted with two isocyanatoalkyl groups and a polythiol (official gazette of JP-A-4-159275) are disclosed.
Although the polymer of item (1) has an increased refractive index by limiting a combination with the polythiol compound to be polymerized, it gives rise to problems that an Abbe number is decreased and a chromatic aberration is increased.
Meanwhile, the resin (2) and the polymer (3) have a high refractive index and the improvement in the chromatic aberration is found therein, but they have a defect that a solvent resistance is poor.
Furthermore, since these prior art polymers are uncrosslinked polymers obtained from bifunctional isocyanate compounds, a special crosslinking agent is needed separately to improve a solvent resistance. Thus, the types of the polythiol compounds that can be polymerized are limited.
In order to overcome many of the problems in the prior art, the invention provides an optical material having the above-mentioned desirable properties. It is a feature of the invention to provide a novel polyisocyanate compound that can give an optical material having a high refractive index, a low dispersion and an excellent solvent resistance and weatherability, and a process for producing this compound at good efficiency. It is also a feature of the invention to provide an optical material which has excellent optical characteristics such as a high refractive index, a low dispersion, an excellent transparency, lack of optical distortion and the like, and which has good solvent resistance and weatherability, and an optical product formed of this optical material.
In particular, the invention provides a polyisocyanate compound having sulfur atoms, contributing to a high refractive index and a low dispersion, in a main skeleton and having three isocyanates as polymerization functional groups. In addition, the invention provides methods for producing the polyisocyanate compound by an efficient processes. It also provides an optical material formed of a poly(thio)urethane obtained by subjecting a component comprising a polyisocyanate compound and a component comprising a compound having two or more of hydroxyl groups or mercapto or both groups to a polyaddition reaction.
In one aspect, the invention provides a polyisocyanate compound having general formula (I) 
wherein n represents an integer from 1 to 4 and each of the indices n is the same.
In another embodiment, the polyisocyanate compound can be produced by one of the following processes: production process 1, production process 2 and production process 3.
Production process 1 of the invention is a process for producing a polyisocyanate compound represented by general formula (I).
Process 1 comprises:
(a) reacting 1,2,3-trimercaptopropane with a halogeno-aliphatic carboxylic acid lower alkyl ester represented by general formula (II)
Xxe2x80x94CH2xe2x80x94(CH2)mxe2x80x94COOR1xe2x80x83xe2x80x83(II)
wherein X represents a halogen atom, R1 represents a lower alkyl group, and m represents an integer of from 0 to 3, to obtain a tricarboxylic acid ester represented by general formula (III) 
wherein n and R1 are as defined above,
(b) converting the tricarboxylic acid ester to a tricarbonyl hydrazide represented by general formula (IV) 
wherein n is as defined above, and
(c) converting the carbonyl hydrazide group into an isocyanate group.
Production process 2 of the invention is a process for producing a polyisocyanate compound represented by general formula (I).
Process 2 comprises:
(a) reacting a 1,2,3-trihalogenopropane with a mercapto-aliphatic carboxylic acid lower alkyl ester represented by general formula (V)
HSxe2x80x94CH2xe2x80x94(CH2)mxe2x80x94COOR1xe2x80x83xe2x80x83(V)
wherein R1 represents a lower alkyl group, and m represents an integer of from 0 to 3, to obtain a tricarboxylic acid ester represented by general formula (III),
(b) converting the tricarboxylic acid ester to tricarbonyl hydrazide represented by general formula (IV) 
wherein n is as defined above, and
(c) converting the carbonyl hydrazide group into an isocyanate group.
Production process 3 of the invention is a process for producing a polyisocyanate compound represented by general formula (I-a) 
wherein k represents an integer of from 0 to 2 and each of the indices k is the same.
Process 3 comprises:
(a) reacting 1,2,3-trimercaptopropane with an aliphatic unsaturated carboxylic acid lower alkyl ester represented by general formula (VI)
CH2xe2x95x90CHxe2x80x94(CH2)kxe2x80x94COOR2xe2x80x83xe2x80x83(VI)
wherein R2 represents a lower alkyl group, and k is as defined above, to obtain a tricarboxylic acid ester represented by general formula (VII) 
wherein R2 and k are as defined above,
(b) converting the tricarboxylic acid ester to tricarbonyl hydrazide represented by general formula (VIII) 
wherein k is as defined above, and
(c) converting the carbonyl hydrazide group into an isocyanate group.
In another embodiment, the optical material of the invention is formed of a poly(thio)urethane, which is obtained by subjecting a monomer mixture comprising component (A) comprising at least a polyisocyanate compound represented by general formula (I), wherein n represents an integer of from 1 to 4, and component (B) comprising at least one type selected from (a) a compound having two or more mercapto groups in a molecule, (b) a compound having two or more hydroxyl groups in a molecule and (c) a compound having one or more hydroxyl groups and one or more mercapto groups in a molecule to a polyaddition reaction.
The invention is to provide an optical product formed of the above-mentioned optical material.